A latching (bistable) relay is a switch which can maintain either an activated state or a deactivated state indefinitely without additional power consumption, and the latching relay consumes power only when the relay is switched. Conventional latching relays often use a permanent magnet to produce part of the magnetic force which is required for the activated state or the deactivated state. Also, the latching relay may have a coil to supply sufficient force for the activated or deactivated state by aiding or opposing the magnetic field of the permanent magnet. Once the relay switches, an additional magnetic field is not required to sustain the switched state. However, the latching relay requires careful magnet level biasing, which can be cumbersome.
Conventional latching reed relays also use the permanent magnet approach to bias the magnetic circuit. A “forward” current pulse through the coil increases the permanent magnet's field strength to activate the relay contacts. Once activated, the permanent magnet maintains the closure. To deactivate the contacts, a “reverse” current pulse is applied to cancel the permanent magnet's field. The contacts are deactivated and remain so because the permanent magnet, in the absence of any coil current, is too weak to activate the contacts.
The selection and placement of that magnet are crucial in the conventional latching reed relay. In some manufacturing environments, graded magnets are selected for a given reed. It can be a problem when attempting to select and place a bias magnet for two reed switches in a double-pole relay. Another approach, using two bias magnets, one for each reed switch, is equally difficult because of magnetic field interaction.